1. Field of the Invention
The present invention relates to a display device, and particularly relates to a liquid crystal display device capable of maintaining brightness even when the screen is high definition and the size of a pixel is decreased.
2. Description of the Related Art
A liquid crystal display device includes a TFT substrate, a counter electrode opposing the TFT substrate, and liquid crystals put between the TFT substrate and the counter substrate. The TFT substrate has pixels including electrodes, thin film transistors (TFT), etc. formed thereon in a matrix form. The counter electrode has color filters, etc. formed at positions corresponding to the pixel electrodes of the TFT substrate. The liquid crystal display device forms images by controlling light transmittance of each pixel using liquid crystal molecules.
Since liquid crystal display devices are flat and light in weight, they are applied in various fields. Small sized liquid crystal display devices are used generally for mobile phones, DSCs (digital still cameras), etc. In the small-size liquid crystal display devices, the size of the pixel is decreased in order to maintain the definition of the screen. Each of the pixels includes, in addition to the pixel electrode, the TFT, a through hole formed for connecting the TFT and the pixel electrode, etc. It is difficult to decrease the size of the TFT and the through hole correspondingly with the decrease of pixel size. Therefore, when the size of the pixel is decreased, the area of the pixel electrode is reduced. The transmittance of a liquid crystal display panel is thus lowered and the brightness of the liquid crystal display device is decreased.
In addition, viewing angle characteristics issue a problem in the liquid crystal display device. The viewing angle characteristics are a phenomenon that brightness and chromaticity differ from when the screen is observed from the front to when viewed from an oblique direction. An IPS (In Plane Switching) system, a system which operates liquid crystal molecules by a horizontal electrode field, has good characteristics in respect of the viewing angle characteristics.
Since, in the IPS system, the pixel electrode and the counter electrode are formed on the TFT substrate side, the problem of the transmittance is further severe. Various kinds of IPS systems exist; for example, a system predominant at current forms a counter electrode as a solid plane, disposes a comb-shaped pixel electrode thereover with an insulating film being put between the two, and generates an electric field between the two electrodes to rotate liquid crystal molecules. This system allows the transmittance to be increased. JP-A-2007-328210 describes a liquid crystal display device using such a system. Incidentally, a system that forms a pixel electrode as a solid plane and disposes a comb-shaped counter electrode thereover with an insulating film being put between the two is similar to the system mentioned above.
On the other hand, JP-A-2009-168878 discloses a configuration such that forms a counter electrode in a layer identical with the gate electrode, and forms a comb-shaped pixel electrode thereover with a gate insulating film and a protective insulating film put between them. According to JP-A-2009-168878, this configuration enables the number of layers in the TFT substrate to be reduced.